Acoustical and thermal liners for application to vehicles are well known in the art. These liners typically rely upon both sound absorption, i.e. the ability to absorb incident sound waves, and transmission loss, i.e. the ability to reflect incident sound waves, in order to provide sound attenuation. They also rely on thermal shielding properties to prevent or reduce the transmission of heat from various heat sources (engine, transmission and exhaust system), to the passenger compartment of the vehicle. Such liners are in particularly used in the engine bay area of a vehicle, for instance employed as an engine cover so as to attenuate the sound of the engine closer to its source.
In the engine compartment of motor vehicles, including passenger and commercial vehicles, sound proofing parts in the form of absorbers are increasingly being used to reduce engine noise. In general, these absorbers are designed as moulded articles to reduce the exterior and interior noise of vehicles. The moulded articles may be made from webs (e.g. cotton) or from polyurethane foam, and typically have thermo stabilities up to about 160° C.
In certain areas, such as exhaust manifolds, hot air recirculation areas or around the engine itself, the moulded articles may be subjected to high thermal loads. Thus these moulded articles are often laminated, partially or completely, with aluminium foil to serve as heat reflectors in order to protect the underlying nonwoven. In general the aluminium foil is thick enough to function as the carrier layer, enabling the mechanical properties for the part to be self-supportive. The sound absorbing material is kept as loose material and as thick as possible to optimise the acoustic properties of the part. For example DE 8700919 discloses such an aluminium laminate with foam glued to the inside for insulating purposes. Other examples are made of sandwiching loose fibrous material mats between two metal foil layers whereby the metal layers do have structural carrier properties.
In recent times composite thermal liners are partly replacing the typical heat shield trim parts. These composite liners are generally formed as multilayer assemblies. These assemblies are build with a thermally exposed layer having reflective and impervious functions, and a composite layer having good thermal insulating, mechanical and structural properties and sometimes with an additional top layer for appearance and imperviousness properties. These types of liners are produced using injection moulding or compression moulding. The disadvantage of these composite thermal liners is that they are impervious and heavy structural parts. Although they have good thermal and structural properties, they lack acoustic and thermal attenuating properties in most of the cases.
While a number of adhesives, adhesive webs and binding fibers have been specifically developed over the years to secure the various layers of the laminates together, laminated liners and insulators have an inherent risk of delamination and failure. This potential risk is significant mainly due to the harsh operating environment to which such liners and insulators are subjected. Many liners and insulators are located near and/or designed to shield hot heat sources such as the engine, transmission and components of the exhaust system. As a result the liners and insulators are often subjected to temperature in excess of 180° C., at which the adhesives or binders show a strong and fast degradation over time.
In addition parts directly mounted adjacent to the engine are likely to vibrate and cause noises due to vibrations transmitted from the engine. These vibrating parts can form an unwanted additional noise. Another aspect is the fatigue properties of the lining involved, the frequency of the vibration can have a negative effect on the overall lifespan of the lining.
A further disadvantage of the state of the art is the high temperature needed to obtain the final composite. The heating temperature to be achieved is dependent on the matrix polymer. In general to form the composite, the matrix and reinforcement fibres are heated using dry heating methods like hot air, contact heating or infrared heating. In order to compensate for the temperature loss for instance from the heating device to the moulding device, the product is normally heated above the true melting point of the matrix polymer or above the activating temperature of the binding resin. Heating of a polymer above the melting point accelerates degradation.
Using a contact heater has the additional disadvantage that the product has to be compressed to obtain a good transfer of heat throughout the thickness of the product. Hot air is generally used at a temperature above the melting temperature of the binder polymer so the polymer gets heat damaged, while the use of infrared heating is only feasible for thin materials. In thicker materials the amount of energy needed to heat the inner material is damaging for the outer surface polymers. This method is normally used only for a thickness up till 4-5 mm.
Using contact heaters in a multilayer lining including an open cell slab foam layer will cause a collapse of the foam in particularly in the skin of the slab foam making it impervious to air borne sound waves, thereby deteriorating the overall acoustic absorption of the part.
Another disadvantage is the fact that most thermoplastic polymers used as matrix fibres and as reinforcement fibres have their melting temperature close to each other for instance the melting temperature as measured using Differential scanning calorimetry (DSC) according to ISO11357-3 of poly ethylene terephthalate (PET) is in the range of 230-260° C., of polypropylene between 140-170° C., of Polyamide-6 (PA6.6) between 170-225° C. and of Polyamide-6.6 between 220-260° C. Using matrix fibres and reinforcement fibres both being thermoplastic polymers, for instance PA6.6 as matrix and PET as reinforcement, having to heat them above the melting temperature of the matrix fibres will also cause the reinforcement fibres to start melting or softening. This will lead to a collapse of the structure, forming a very compact composite.
Felts are widely used particularly in automotive industry for their thermal and acoustic insulation properties. The trend is towards recyclable materials; therefore thermoplastic binders have taken a significant share in the last years. Fibers made of high performance polymers such as polyesters, polyamide are highly interesting due to their mechanical and heat resistance properties. But the necessary binding agent form the limitation to their utilization in moulded 3D parts.
The binding agents used so far always have a lower melting point than the reinforcement fibres, rendering in relatively weak performance behaviour to the moulded fibre web and limiting its utilization to tempered areas in the vehicle. None of these types of moulded fibre webs is suitable for the high temperature exposure of the engine bay or compartment, particularly of the engine contact areas. Some of these binders are modified polymers (Co Polyester (CO-PET) as an example) having pour behaviours due to their modified structure being particularly sensitive to hydrolysis phenomena.
The processes for moulding such felts as known in the state of the art are a ‘cold” moulding process where the felt is pre-heated by various means, and then transferred to a cold mould in which it is compressed in order to obtain the part shape or a ‘hot” moulding processes, where the felt is introduced in a closed mould, in which a heat transfer media, like air, is introduced for bringing the binding agent to its melting point, and then released. The part is then cooled down, inside the tool or outside, with or without cooling assistance. (See for instance EP 1656243 A, EP 1414440 A, and EP 590112 A) Only after complete cooling down to a temperature at which the material is set, the part can be taken out of the mould and transported.
Fibrous composites as disclosed are generally used in combination with additional layers, like the reflective layers as discussed or with foam. Foam can be applied to such fibrous composites by direct back foaming (injection foaming or foam moulding) the foam. However often the foam is first produced as slab foam and cut into the thickness desired. For the lamination of the foam to adjacent fibrous layer generally hot compression moulding is used. The stack of layers is put between two hot plates to melt the material and obtain a lamination of the layers. Compression is needed to help the transfer of heat to the porous reinforcement of the layered material. A disadvantage of such a method, in particularly where foam layers are used, is that the foam collapses and forms a skin layer on the open cell structure. This skin layer deteriorates the overall acoustic absorbing performance of the open cell foam.